Are there devices which can figure out at what frequency every device transmitting radio frequencies operate at precisely? Say from a high of 2.4 GHz microwave to a low of 402 Hz pacemaker.


closed as off-topic by Chris Stratton, Voltage Spike, Finbarr, Sparky256, RoyC Mar 28 at 7:57

  • This question does not appear to be about electronics design within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.


Yes, such devices exists and are called spectrum analyzers.

You will also need to buy an near field probe kit, if you want to work with lower frequencies. They do not propagate well through air, so you need a somewhat sensitive "antenna" that must be put near the source.

The range of frequencies you need is quite wide, be ware that not all makes/models support this, and it is going to cost you a few 10 k$.

  • 1
    \$\begingroup\$ If the interesting frequencies are that far apart, it might even be sensible to look at two bits of equipment instead of one. \$\endgroup\$ – Attie Mar 22 at 11:09
  • \$\begingroup\$ We have a Tek that does cover that frequency range, but we had to purchase the "low frequency option" in order for it to be able to go below a few 10 MHz (if I recall correctly) \$\endgroup\$ – Vladimir Cravero Mar 22 at 12:51
  • \$\begingroup\$ Interesting, I thought most things that reach up to GHz don't reach far below the low-kHz range. \$\endgroup\$ – Attie Mar 22 at 13:03
  • \$\begingroup\$ RSA5103B, 1 Hz - 3 GHz. And that's the base model, the top seems to be from 1 Hz to 26.5 GHz. Don't ask me the price tag though... I imagine it to be quite high. \$\endgroup\$ – Vladimir Cravero Mar 22 at 13:55
  • \$\begingroup\$ Hah, shiny, thanks! \$\endgroup\$ – Attie Mar 22 at 18:12

As Vladimir said, what you're looking for is a spectrum analyzer.

Whether you actually need a high-quality, high-sensitivity measurement device or just something that can observe a bit of spectrum, move on to the next and give you a rough idea of what's there in power, can't be answered from your question.

As Vladimir also pointed out: at 402 Hz, you're not seeing an electromagnetic wave emanating from your device (you're too close, compared to wavelength), so no "RF", since there's no "R" for Radio like Radiating. (A somewhat feasible antenna for 402 Hz radio waves would literally be hundreds of km in length). You'd more be looking at one half of a transformer than at an antenna! So, you need a near field probe (which happens to be technically a winding, like a transformer winding). (also, at all the other sources of low-frequency EM).

If you can actually work with starting at higher frequencies (say, 60 MHz) and using an oscilloscope with a few copper windings for anything below as improvised near field probe / detector:

A couple hundred euros in SDR equipment (example, I'm affiliated with these guys) could give you continuous coverage from 60 MHz to about 6 GHz, given you add broadband antennas.

That equipment wouldn't be calibrated, so you won't get absolute powers, but would still be able to infer relative emission levels.

I'd like to add that EMI measurements require you to make sure it's mainly your device under test doing the emissions – the cost of an RF-isolated measurement chamber might be significant, depending on how sure you need to be.

If you go the uncalibrated SDR route – which might be very fine for you, because your question suggests you're more interested in frequencies than exact power levels – notice that you might want have a high-quality oscillator. These could be the same oscillator used to drive the device under test, or another measurement device, or a GPS-disciplined oscillator: for some SDR devices (the example I linked to above, for example) these are available as add-ons.

You can get excellent frequency estimation performance with an SDR – better even than with a classical Spectrum Analyzer – if you know what you're looking for.

  • 1
    \$\begingroup\$ For pickup of very low frequencies, just use a piece of copper foil, perhaps the plane of a PCB; this responds to Electric fields emitted from even the smallest of circuits; use a 10MegOhm scope probe for pickup, or an OPAMP in unity-gain configuration. \$\endgroup\$ – analogsystemsrf Mar 22 at 9:58
  • \$\begingroup\$ that'd be an E-Field antenna; my guess is that if someone communicates with a in-vivo pacemaker, it's going to be through a magnetic field, hence my loop. \$\endgroup\$ – Marcus Müller Mar 22 at 10:16
  • \$\begingroup\$ @ Marcus At 402 Hertz, its not RF, because you cannot dodge the near-field. The OP will see lots of Efield radiators. \$\endgroup\$ – analogsystemsrf Mar 22 at 17:30
  • \$\begingroup\$ @analogsystemsrf good point; corrected. \$\endgroup\$ – Marcus Müller Mar 22 at 18:12

For low frequencies, do this


simulate this circuit – Schematic created using CircuitLab

You will see lots of 60Hz (50Hz, in Japan) on the output.

Near a loudspeaker cable, you will see the MUSIC waveforms.

Near a car, you see the spark plug voltages, and probably the alternator.

Near a black-brick battery charger, you find the switching power supply at 100,000Hz and up.

Near a computer display, you see the 100,000 Hertz???? row update rate.

  • \$\begingroup\$ OA1 = ???...... \$\endgroup\$ – AnalogKid Mar 22 at 14:00
  • \$\begingroup\$ @AnalogKid Operational Amplifier. Was that your question?! \$\endgroup\$ – Marcus Müller Mar 23 at 17:31
  • \$\begingroup\$ What is the part number for OA1? \$\endgroup\$ – AnalogKid Mar 23 at 19:30

Not the answer you're looking for? Browse other questions tagged or ask your own question.